Today, just over five years since his body was found inside a padlocked bag, his death remains one of Britain’s most mysterious unsolved cases.

The Sun on Sunday can reveal that voicemail messages Mr Williams left for family and pals were deleted in the days after his death. And a rival agent may also have broken into the flat to destroy or remove evidence.

The inquest was barred from discussing Mr Williams’s work in public. But sources say he was helping on the joint monitoring network Echelon, which uses sophisticated programs to eavesdrop on terrorists and criminal gangs, particularly those in Russia...